fix(prometheus): the list page is a section top; the scrape test builds its text without format(); docs (#5668, #5670)

- tests/test_breadcrumbs.py: the Prometheus target list is its section's
  own top page, like SNMP's, so it carries no trail.
- The integration test's exposition has label braces, which str.format read
  as fields: the fake scrape raised, the target was recorded down and no
  series landed. It substitutes the counter with replace() now.
- docs/plugins/prometheus.md: adding a target, filtering, how series are
  named and stored, alerting, and the Fabled Archives recipe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/integration/test_prometheus_scrape.py

@@ -16,7 +16,7 @@ TEXT = """\
 # TYPE fa_backup_target_state gauge
 fa_backup_target_state{target="Steward",state="late"} 0
 # TYPE fa_requests_total counter
-fa_requests_total {count}
+fa_requests_total COUNT
 # TYPE noise gauge
 noise 1
 """
@@ -63,7 +63,7 @@ def test_a_scrape_records_series_rates_and_a_down_target_fires_its_rule(app):
         tokens.append(token)
         if "gone" in url:
             raise ScrapeError("Couldn't connect: connection refused.")
-        return prometheus.parse(TEXT.format(count=next(counts)))
+        return prometheus.parse(TEXT.replace("COUNT", str(next(counts))))
 
     async def latest(name_prefix):
         async with app.db_sessionmaker() as s: